Safety quality grading evaluation on the external coating defects of buried steel pipeline

The evaluation method on pipeline external coating grading especially the defects of external coating in China has not been established. Aiming at this situation, the applicability and grading index reasonability in grading standards from other countries are analyzed in the paper. The analysis results reflect the differences existed in the grading standards, and based on systematic comparison, the grading evaluation indices which are more practical to application in China are proposed. Through putting the emphasis on the ACVG inspection principle and establishing the ACVG defect electrical field model, three indices in current level difference, radiation distance and approximated electrical field energy are put forward. Based on large scope field excavation and statistics study, the applicable external coating defect grading evaluation regulation is provided. According to the regulation, the field investigation is carried out. Results show that the result similarity reaches 88.24% for indirect and direct inspection, proving the accuracy and applicability of the grading evaluation regulation. By the grading, pipeline operator can make corresponding strategy to enhance the pipeline repair efficiency and pertinence according to different grades.